In recent years, metallocene compounds have been known as homogeneous catalysts for olefin polymerization. With regard to methods for polymerizing olefins (in particular, methods for polymerizing α-olefins) by use of metallocene compounds, since isotactic polymerization has been reported by W. Kaminsky et al., a lot of improvement studies have been conducted for improvement of stereoregularity and polymerization activity (Non-Patent Document 1).
In α-olefin polymerization by use of metallocene compounds, it is known that the stereoregularity and molecular weights of resultant olefin polymers greatly vary by the introduction of substituents to the cyclopentadienyl ring ligands of the metallocene compounds or by the crosslinking two cyclopentadienyl rings.
For example, when metallocene compounds having a ligand in which a cyclopentadienyl ring and a fluorenyl ring are crosslinked with each other are used as a polymerization catalyst for propylene, in terms of the stereoregularity of polymers, use of dimethylmethylene(cyclopentadienyl) (fluorenyl)zirconium dichloride gives syndiotactic polypropylene (Non-Patent Document 2); use of dimethylmethylene(3-methylcyclopentadienyl) (fluorenyl)zirconium dichloride, which has a methyl group attached to the 3 position of a cyclopentadienyl ring, gives hemi-isotactic polypropylene (Patent Document 1); and use of dimethylmethylene(3-tert-butylcyclopentadienyl) (fluorenyl)zirconium dichloride, which has a tert-butyl group attached to the 3 position of a cyclopentadienyl ring, gives isotactic polypropylene (Patent Document 2). A metallocene compound is also being considered in which some of the hydrogen atoms of a cyclopentadienyl group that is a ligand portion of the metallocene compound are substituted with alkyl groups (Patent Document 3).
In modifying these metallocene compounds, it is possible to obtain relatively high melting points, an indicator of the stereoregularity of a polymer, and to obtain sufficiently high molecular weight. Also, it is possible to produce a polymer with a high melting point and a sufficiently high molecular weight.
As an example of producing a poly-1-butene using such a metallocene catalyst as described above, a polymerization example with an ethylene bis(indenyl)zirconium dichloride is described in Non-Patent Document 3, but there is a problem such that a poly-1-butene obtained by polymerization using such a catalyst has a low molecular weight and the catalyst has a low polymerization activity as well.
As a catalyst capable of producing a poly-1-butene having a high molecular weight, Patent Document 4 describes a catalyst for olefin polymerization that is composed of a metallocene compound and an aluminoxane, but there is a problem such that a resultant poly-1-butene has a low molecular weight and the catalyst has an insufficient polymerization activity.
As an example of producing a poly-1-butene using another metallocene compound, an olefin polymerization catalyst composed of a metallocene compound and an aluminoxane described in Patent Document 5 and an olefin polymerization catalyst composed of a metallocene compound and an aluminoxane described in Non-Patent Document 3 are reported, but there is a problem such that both of them have an insufficient polymerization activity.